1. Field of the Invention
The present invention relates to a communication system providing voice and data communication services through LAN (Local Area Network) system and relates to a cordless communication system of a private branch exchange (PBX) system wherein plural radio base stations are present in a limited are.
2. Description of the Related Art
Generally, a personal computer which is equipped with a voice processor board and a network I/F (Interface) card is provided for managing a voice communication or a data communication of a PBX (Private Branch eXchange) system within a limited area of one firm location, that is, the voice processor is connected to a PBX switchboard through a telephone line and the voice communication is performed through the PBX switchboard. A wired network system such as LAN (Local Area Network) system is connected to the network I/F card and the data communication is practiced among personal computers connected to the network system. However, in this communication system, a large-scale wiring construction is necessary for the system to connect all personal computers which handle telephones provided for each persons to a telephone network. Furthermore, a massive PBX switchboard is necessary to assign one telephone line per person. Moreover, a telephone conversation is limited within the telephone network, when a voice processor board which is applicable for the telephone network is installed. In any cases, a call can be initiated only by a private line. Therefore, it is impossible to initiate or to answer to a call unless a person is on his private line, although the massive PBX switchboard is equipped or the voice processor board which is suitable for the network is installed.
In addition thereto, mobile radiotelephones such as PHS (Personal Handy-phone System) telephones are used for a communication system of a PBX system wherein the PHS telephones communicate with other telephones through radio base stations. The communication system is used in firms in conjunction with the LAN system. When a PHS telephone is moved from a service area of one radio base station which is hereinafter called an original station to another service area of another radio base station which is hereinafter called a new station, the PHS telephone receives a control signal from the new station and notifies the new station that the PHS telephone can communicate with the new station so that the PHS telephone can not receive a control signal from the original station any more. Service areas of plural radio base stations are assigned for overlapping partially or intentionally assigned to overlap in heavy traffic areas. All communication channels may become busy although plural communication channels which are called an individually assigned slot are provided for one radio base station. In this case, a mobile radiotelephone would not search a control signal from the other radio base station unless the mobile radiotelephone is moved into other service area while the mobile radiotelephone is receiving the control signal from the original station. Therefore, the mobile radiotelephone can not communicate with other radio base stations as far as staying in the same service area when all communication channels of the original station from which the mobile radiotelephone is receiving the control signal are occupied by other radio equipment although some communication channels of another radio base station which can be communicated with the mobile radiotelephone within the same service area are vacant. The mobile radiotelephone is obliged to keep waiting for a vacancy of the communication channel of the original station. Furthermore, the mobile radiotelephone searches the same original station once again and can not communicate with other radio base stations although a power switch of the mobile radiotelephone is turned off and turned on to search another radio base station due to a same process algorithm for searching radio base station.